The strut type wheel suspension system is widely used in automobiles. Such a wheel suspension system typically comprises a wheel carrier, a link member joining the wheel carrier to the vehicle body, a tubular shock absorber including a cylinder filled with oil and a piston slidably received therein extending approximately vertically between the wheel carrier and vehicle body and a coil spring surrounding the shock absorber. The shock absorber receives a force input from the wheel carrier, but the direction of this force input typically does not coincide with the axial line of the shock absorber. The resulting lateral force component produces a lateral force and a moment at the sliding part between the piston and cylinder, and this in turn causes a frictional resistance between them. This not only impairs the ride quality of the vehicle, but reduces the service life of the shock absorber.
To mitigate this problem, it has been proposed to mount the coil spring in such a manner that the axial line of the coil spring is angularly offset from the axial line of the shock absorber. The lateral force and moment which are produced by the coil spring and applied to the shock absorber can be used for canceling those produce by the force input from the wheel carrier. However, the possible amount of offset is dictated by the diameter of the coil spring and the available mounting space, and it is difficult to produce adequate amounts of lateral force and moment from the coil spring.
Japanese patent laid open publication (kokai) No. 1-156119 discloses a coil spring having a curved axial line in its free state. It is discussed that the curved coil spring produces a lateral force and moment that can cancel those produced at the sliding part between the cylinder and piston of the shock absorber. It is however not clear how this curved coil spring can be mounted and retained in a straight disposition. Producing such curved coil springs on a commercial basis would be difficult because cost considerations are highly important for coil springs for vehicle suspension systems.
Japanese patent application No. 2000-562229 by the assignee of the present application discloses a coil spring having a vertical and straight axial line and a pitch angle that varies in each turn in a cyclic manner. This coil spring has a cylindrical profile, and can be installed in a limited space. Furthermore, it can be easily installed as there is no need to support any lateral force when installing it. This coil spring can be most conveniently produced in a hot coiling process, but there is some difficulty in producing it in a cold process which involves the use of feed rollers.
Japanese patent laid open publication No. 2002-35877 discloses a method of manufacturing an oblique coil spring suited to be produced in a cold coiling process. Although the coil spring has an oblique but straight axial line, a lateral force is produced as it extends and compresses. It therefore can be favorably used in a vehicle wheel suspension system. However, the proposed method is an attempt to form an oblique coil spring simply by adjusting the turn radius (radius of curvature) of the coil spring.
However, according to the conventional coiling machine, as the radius of curvature of the coil spring changes from one point to another, the lateral distance between the roller for controlling the outer diameter and a tool for controlling the pitch changes, and this causes the pitch angle of the coil spring to be uneven from one point to another in each turn. Such a variation in the pitch angle causes the pitch at each point of the turn to be uneven, and this unevenness accumulates over the length of the coil spring with the result that the axial line of the coil spring tends to curve significantly.
Even when the axial line of the coil spring is curved, it still can produce a lateral force as it extends and compresses. However, such a coil spring would require a relatively large mounting space as was the case of the coil spring mounted with an offset with respect to the axial line of the shock absorber, and would be unable to produce an adequately large lateral force.